A typical resonant circuit fluorescent lighting ballast and fluorescent lamp are shown in FIG. 1. Operation may be understood by representing this circuit as two equivalent resistor-inductor-capacitor (RLC) circuits. The first equivalent circuit, shown in FIG. 2, is series resonant at a particular frequency, typically about 70 kHz, the series resonance of the inductor 710 and the filament capacitor 716 (Cf). The second equivalent circuit is shown in FIG. 3. Note that in both equivalent circuits the capacitor 714 (C) has been replaced by a short circuit (zero resistance). The function of the capacitor 714 is to perform DC blocking (allowing only AC signals through the circuit) and is chosen to have a high value of capacitance for this purpose. It is modeled to be a short (low impedance connection at the AC signal frequencies) in these equivalent circuits.
When the fluorescent lamp is off, the ballast is first driven at frequency, FHigh. This frequency is chosen to be above the resonant frequency point of the RLC circuit, and is typically about 100 kHz. At this frequency, FIG. 2 best represents the lamp's equivalent circuit since the lamp gas has not yet ionized. The frequency response of the circuit with respect to the current is shown in FIG. 4. The purpose here is to run current through the filaments of the lamp, this is typically referred to as the ‘Preheat’ interval. When the filaments are warm enough to ionize the surrounding lamp gas, the drive frequency is lowered. This causes the RLC circuit to be swept through its resonant frequency, causing an increase in the voltage across the lamp. An arc will occur in the lamp at its ‘strike’ voltage and the arc will ignite (ionize) the gas.
Lamp ‘ignition’ means that the gas is now ionized enough to conduct an electric current. The lamp is now said to be on (producing visible light). At this point, FIG. 3 best describes the behavior of the lamp ballast circuit. Note that the lamp now behaves as an L in series with a parallel R and Cf. The R in this case is the electrical resistance of the ionized gas in the lamp and Cf is the filament capacitance 716. The frequency response of the circuit with respect to lamp current is shown in FIG. 5. Note that while the gas in the lamp is ionized, the current increases as the drive frequency is decreased. There is a point on the frequency response curve where the current is pinched off. Note that this point can be selectable by the ballast designer by manipulating the values of L and Cf.
While the lamp is on, it will be driven at a frequency, FLow. The ballast designer may choose this drive frequency as optimal for the specified wattage of the fluorescent lamp. If the drive frequency is increased, that is the RLC circuit is de-tuned, the lamp will start to dim. As FIG. 5 shows, the current though the gas in the lamp will decrease and so the light output will decrease with the decrease in current. As the drive frequency is increased, at some point between FLow and FHigh, the lamp will go out as the lamp current gets ‘pinched’ off.
There are a number of state of the art analog techniques in the literature and on the market that make use of the above mentioned effects. Dimming is accomplished by modulating the drive frequency to the RLC circuit.
The industry standard method of modulating the drive frequency is with an analog voltage controlled oscillator (VCO). A DC voltage is fed into the modulator input of the VCO and a square wave signal is generated. The device identified as ‘Logic Block’ in FIGS. 1 through 3, converts the square wave into two drive signals on the gates of the power MOSFET transistors. A typical implementation of this circuit is shown in FIG. 6.
The frequency resolution of the VCO is important. FIG. 5 shows that the relationship between the drive frequency and the lamp current is not linear, rather it is more in the shape of an ‘S’ curve. This makes the light output response of the lamp difficult to control without the use of more sophisticated circuitry. Many implementation of this sort of control system are on the market today.
Note that the steepest slope on the curve is close to its ‘pinch off’ point (around 60 kHz in FIG. 5). In this frequency band, small changes in frequency yield large changes in brightness. The method of dimming the lamp in this classic fluorescent lamp resonant circuit involves modulating the drive frequency. That is, as the frequency is raised linearly, the lamp brightness is lowered exponentially. This effect is not tolerant to coarse frequency modulation signals, especially at these low brightness levels. If the granularity of frequency control is too large, stepping from one frequency to another will result in a very visible brightness change; i.e., the lamp brightness is quantized.
Another challenge to the classic analog drive methods occur on all dimming ballast circuits at low brightness levels. The filaments of the lamp need to stay warm so as to ionize their surrounding gas. When little current flows through the lamp, the filaments cool and the lamp goes out. More complex drive circuits are needed to provide DC (or AC) bias to the filaments to keep them warm and thus compensate for this effect. There are many examples of this type of compensation in the literature. They all tend to add more components and complexity to the ballast design.
Feedback control is needed with this circuit solution. Whenever the lamp's temperature changes, its luminescence changes. So at a particular, constant drive frequency, the lamp brightness will vary until it reaches thermal equilibrium. A feedback control loop is typically employed so as to monitor the lamp current. As the lamp temperature changes, so will the current through the lamp. The drive frequency is adjusted continuously so as to maintain constant brightness, e.g., constant lamp current.
A much worst effect can also happen on cool filaments leading to their premature failure. When the current through the lamp is low, a ‘hot spot’ can develop on a filament. The lamp current will concentrate its flow into this small area on the filament where the gas is well ionized. Continued, differential, thermal stress on this small area of the filament can cause an open circuit there. Running current through the filament will evenly heat the entire filament, and thereby distribute the lamp current across the filament's entire length. Since all of the filament will be hot and have ionized gas around it, lamp current will not concentrate at any small spots.